The proposed research addresses molecular mechanisms controlling cell proliferation, with an emphasis on the members of the p21 or CIP/KIP family of CDK inhibitors. The proposal includes three diverse aims. The first aim of the proposal will be to pursue mechanisms controlling the stability of these inhibitors. In particular, the applicant will continue ongoing studies of a recently identified protein, known as ROC1, which appears to play a role in the ubiquitination of G1 regulatory proteins. The investigator proposes to use biochemical approaches to characterize the role of ROC1 in the degradation of CIP/KIP inhibitors, with an emphasis on the identification of proteins involved in targeting ROC1 to these inhibitors for ubiquitin-dependent degradation. In the second aim, the applicant will analyze the phenotypes of mice bearing double mutations in different CDK inhibitors, with the goal of unveiling overlaps in the function of different inhibitors in vivo. The third and final aim will focus on the control of cell proliferation by the p53 regulator, ARF. Preliminary results indicate that ARF associates with p53 and another p53 regulator, MDM2, in concentrated "nuclear bodies" in the nucleus, and the investigator proposes to clarify the function of these bodies by searching for ARF-associated proteins and by purifying and identifying the components of these nuclear bodies.